Transportable VHF repeater serves Antarctic communications Repeaters, deployed by helicopter for summer communications requirements and retrieved for winter storage, avoid temperature stresses that destroyed components and batteries in previous permanent

In the Dry Valley region of the Antarctic Transcontinental Mountain Range, peaks soar more than a mile above winding valleys containing field party camps. Scientists from the National Science Foundation (NSF) and other workers in these camps endure mean annual temperatures of -17.7 C and average winds of 13mph, with low temperatures of -50 C and maximum wind speeds higher than 115mph. Survival in this harsh environment requires dependable communications with McMurdo Station on Ross Island more than 50 miles across the Ross Ice Shelf. Helicopters provide the only access to these field camp sites, and high- frequency (HF) or very-high-frequency (VHF) transceivers provide the only communications with the aircraft and McMurdo Station. Rough terrain and ionic interference characteristic of the polar regions often make HF communications difficult. VHF transceivers have proved to work well when the line-of-sight propagation limitation is overcome with mountaintop repeaters. In past years, VHF communications problems were addressed by using a permanent mountaintop structure containing a VHF transceiver and a UHF transceiver set up as a crossband repeater. The system included storage batteries, a solar charging unit, antennas and a solar array. This permanent installation had three problems. First, because the structure was permanent, but not powered for heat during the long, sunless winters, all equipment that could not survive the -40 C to -50 C cold storage had to be dismantled and returned to McMurdo Station for warm storage. Re-installation at the start of the next season was time-consuming and sometimes was interrupted by bad weather. Second, a person at one field camp could not talk directly with someone in another field camp. The use of a VHF (field party) to UHF (McMurdo station) crossband repeater required that all field party communications be routed through the U.S. Navy communications center at McMurdo station, sometimes resulting in a communications choke-point. Third, the solar arrays did not work well because they were installed flat in a plane parallel to the ground. (See Figure 1 on page 38.) The sun never sets during the field party season and rotates through the compass points in a sinusoidal pattern, with a low point of about 78 degrees to a high point of 55 degrees from the vertical (at mid season). Since the efficiency of most solar cells drops off greatly at an incident angle of more than 20 degrees from the cell vertical, the installed solar array was operating at a small fraction of rated capacity. Frequently, the old repeater battery bank became discharged, raising the freezing point of the batteries. Frozen batteries failed because of warping and required mountaintop replacement.

Portable repeater To support the Special Communications Engineering Branch of the Naval In-Service Engineering Center, East Coast (NISE East), my co-workers and I developed engineering design solutions for these problems. The result is model OAE-1 (for Old Antarctic Explorer), a self-contained, multipurpose repeater that is transportable by helicopter. System components withstand and operate in temperatures as low as -40 C. The repeater supports multiple communications channels and provides a platform for auxiliary (meteorological) equipment. The open-frame structure has solar panels on all sides and an equipment case centered on the base for stability. (See Figure 2 on page 41.) The structure is composed of an aluminum tubular frame, four structure-leveling jacks, four deployable outriggers, equipment shock protection and an ECS Containers equipment case. This open-air aluminum tubular frame has a small cross-section to minimize wind loading. The frame provides support, mounting and protection for all system components during storage, deployment and shipping. The frame base contains mounts for the leveling jacks, the deployable outriggers and the equipment shock absorbers. The equipment case contains a double-sided, double-ended, 19-inch equipment rack with insulation to reduce heat loss for optimum cold weather resistance. The repeater channels, the solar power subsystem (except for the solar panels), the remote control subsystem, the monitor subsystem (except for the wind sensors) and the RF distribution subsystem (except for the dipole antenna) are all contained in the equipment case. The antenna is a wideband, halfwave, omnidirectional dipole. The system can be assembled easily and flown to the repeater site. At the site, the repeater is unhooked from the lifting rigging, the antenna and accessory cables are installed and connected, wind braces are rigged and the repeater is turned on. The system is then in operation and ready for use immediately. The basic repeater channel is composed of equipment from Daniels Electronics, Victoria, British Columbia, Canada, including a VR-2 receiver; VT-2 transmitter; VT-30 power amplifier; an audio control card with hang-timer and DTMF capability; and an MT-2 repeater chassis. The antenna subsystem provides RF signals in the 138MHz-139MHz band to the VR-2 receiver via coaxial cable, where the audio signal is demodulated and sent to the MT-2 chassis backplane, along with a carrier-operated squelch (COS) signal. The audio control card takes the RX audio and COS signals from the backplane, processes the signals for RX/TX gain, muting and push-to-talk (PTT) capability, and provides TX audio and PTT signals to the chassis backplane. The VT-2 transmitter takes the TX audio signal from the backplane and modulates the transmit (143MHz-144MHz) RF signal when a PTT signal is present. The TX RF signal passes to the VT-30 power amplifier via coaxial cable. When the PTT signal is present on the chassis backplane, the VT-30 boosts the RF signal to the desired level for transmission and sends it to the antenna subsystem. Figure 3 below shows the signal flow for one VHF repeater channel.

Repeater signal flow The repeater channels have four major support subsystems: (1) solar power subsystem. (2) RF distribution subsystem. (3) monitor subsystem. (4) control subsystem. The solar power subsystem is composed of four MSX-83 Solarex 83W solar panels, a NDP-30-TC Sun Selector 30A charging controller, two adjustable LVD-8 low voltage disconnects and six Dynasty model GC12V100 sealed lead-acid (gelled) batteries. Each solar panel (side-mounted, to maximize sunlight incidence) provides power through blocking diodes to the charging controller and then to the storage batteries and the 12Vdc bus. Each VHF repeater channel receives power through separate low voltage disconnect units. A third circuit provides power to auxiliary modules, including the multicoupler, the Omega data logger, the Young wind speed-and-direction interface and the DTMF and repeater control unit. The two repeater circuits and the auxiliary circuit can be isolated for maintenance or storage by switches on the power distribution panel. The charging controller has a temperature sensor to regulate the charge rate of the batteries. The RF distribution subsystem is composed of a VHF Sinclair SPL-210C omnidirectional dipole antenna, two Q-2222E Sinclair 3MHz spaced duplexers and a RM-21202N Sinclair multicoupler. The RX RF energy received by the dipole antenna is routed through the RX/TX duplexer to the multicoupler where it is amplified, split and sent to the VR-2 modules of the two VHF repeater subsystems. TX RF energy from the VT-2 modules of the two VHF repeater subsystems is combined in the TX1-TX2 duplexer. The combined TX signal is routed through the RX/TX duplexer to the dipole antenna for transmission. Figure 4 at the left shows the signal flow for the RF distribution subsystem.

Antenna subsystem signal flow The monitor subsystem is composed of an Omega OM-220-EXM-1 data logger, three OM-220-VIM-2 voltage input modules, an OM-220-RTDIM-2 RTD input module, two PR-11-2100-1/4-6-E sheathed RTD 1/4″ temperature probes, a Young wind speed-and-direction transducer and a Young signal converter. The Omega data logger is the heart of the monitor subsystem, sampling as many as eight analog signals and storing them as digitized data for transmission on request. The sample times and sample values are stored in memory for transmission via modem through the repeater controller for the control channel. This transmission function is activated through DTMF codes that alter the audio path, replacing the repeater circuit with a full-duplex data circuit to the Omega data logger. The remote control subsystem is composed of a DTMF decoder, circuit control relays, a PacCom 2400 radio modem and a base control station. The audio controller for the repeater subsystem has DTMF control functions built into the audio controller. These DTMF functions include mute control, repeater disable, repeater master number assignment, auxiliary outputs, data transfer mode and other functions. Of the many DTMF functions available, there are two main types used by the repeater system: Repeater control functions — Squelch, mute and power level control for both channels, including master first number (when more than one system deployed). Data transfer function — Used to shift the field operations repeater to data mode for interface with the Omega data logger. The base control station, a self-contained, full-duplex, stand-alone transceiver, located at a site common to all deployed repeater systems, can be used for several functions: Communications — Both repeater and talk-around communications on both channels. Repeater control — Squelch disable, channel disable and power level adjust for both channels, shifting to data mode. Repeater programming — Used for entering or changing repeater DTMF codes, enabling or disabling DTMF functions. When the base control station is used in data mode, a personal computer with Omega data acquisition and control systems software is required, along with a serial interface to the base station.

Summary The first season (1993-1994) that this particular design for the VHF repeater system was used, it was deployed in repeater mode without DTMF capability to test area range, cold weather survivability, power system operation and system reliability. During the season, the old repeater systems failed and could no longer be restored. One of three transportable repeaters was placed on the crater lip of Mount Erebus, an active volcano and the highest point on Ross Island. A second and third were placed on Mount Newall and an unnamed peak in the Dry Valleys. From these vantage points, the repeaters performed flawlessly and ended up carrying both aircraft and field party communications for the remainder of the season. At the end of the season, the system was retrieved and placed in winter storage for a well- deserved rest.